The invention is useful in measuring the time delay imparted to a signal as it propagates through an integrated circuit (IC). This testing is measured primarily in digital switching integrated circuitry. When a new IC logic family is developed, very extensive data must be taken to document the DC and AC parametric characteristics prior to the IC's being useful in constructing digital devices. Data must be taken, for example, on the effect of variations of power, temperature, and humidity as well as storage, vibration, and radiation. In the past DC parametric testing has been reasonably performed. AC parametric testing has presented a greater challenge. It has either been done in a laboratory setting with very expensive testers or it has been done laboriously on the bench with oscilloscopes, fixtures, pulse generators, power supplies and the like. In all cases, the testing was slower, more expensive and generally not as accurate as the instant device. This invention permits rapid testing of many thousands of IC's, recording of the results, and subsequent introduction. Of a variable such as previously mentioned. Additionally, all the testing can be easily reperformed. Further, the invention makes possible, for the first time, on-line automated AC parametric production testing of many IC's.
The invention is an improvement to a delay lock loop as detailed in U.S. Pat. Nos. 4,309,673 and 4,338,569. The disclosure in those patents is identical. Accordingly, the following remarks are applicable to both patents. No ramp generators were incorporated in the '673 and '569 device. Simply, the rise time of a square pulse was utilized as the ramp. As is known, such pulses appear square when presented as a pulse train on an oscilloscope. In fact, however, there is a finite amount of time required for the pulse to rise. This can be seen very graphically on an oscilloscope by selecting a short time base.
In the embodiment shown in the two cited patents, the pulse was not variable with respect to time. Accordingly, the rise time of the pulse was fixed. Consequently, the range of time was fixed. No range switching was possible. The result was that only delays that were less than the fixed rise time could be measured. This imposed a severe limitation on the utility of the device, at least with respect to the testing of IC's. Further, the rise time was extremely fast being measured in picoseconds whereas the instant device has delay ranges extending out to 1000 nanoseconds.
The rise of the pulse in the '673 and '569 device does not exhibit the high linearity that is required in performing the precise measurements demanded in the application of the instant invention. The slope of the rise is shallow initially, building to a steeper gradient as the rise progresses.